Spring-ai Fix OpenAI ChatResponse usage calculation when toolcalling is used

• Fix OpenAI ChatModel's call() operation

• When toolcalling is used, calculate cumulative usage from the preceding ChatResponses

• Fix OpenAI ChatModel's stream() operation

• Make sure that cumulative usage is calculated from the ChatResponse which has a valid usage

  • Use overlapping buffer to check and store the usage from the response that holds the usage.

• Add tests for both call() and stream()

Comment From: ilayaperumalg

This was merged as 37cf20dfe4679a17e96759c139b4ba3ec765fe9c